Generally, a sheet heater for vehicles is maintained at a constant temperature by supplying strong electric current to the heater through a thin electric wire to elevate the temperature of the heater to a desired temperature and controlling supply of electric current to the heater using a temperature sensor or bimetal regulator. However, this product is likely to undergo heat loss due to power interruption relating to disconnection of the electric wire or emission of heat from the electric wire and has low uniformity of heat generation due to manual arrangement of the electric wire.
Most sheet heaters for vehicles are designed to operate on 12 volts. When such a sheet heater is made of an existing carbon paste, the carbon paste is printed in a net shape to prevent local temperature increase and a silver paste for electrodes is used to form four or more wires in consideration of resistance variation according to distance and disconnection between the carbon paste and the silver paste, thereby limiting product size. Therefore, the existing carbon paste is not suitable for production of sheets type heaters having a size of more than 250×300 mm and operating on 12 volts and provides low thermal durability to a final product due to non-uniform heat generation.
FIG. 1 is a diagram of a heating mechanism of a conventional wire heating element. In this heater, since a contact surface between a heating wire and an object is limited, such heater exhibit poor heat transfer to an object to be heated and slowly rises to a maximum operating temperature.
FIG. 4 shows an electric network structure of general carbon. For electric conduction through general carbon, carbon is partially mixed with metal with a binder to adhere particles to one another. Thus, when disconnection between the particles occurs, electric current is concentrated on a certain portion where the disconnection does not occur, so that heat is generated from the certain portion, causing disconnection of the certain portion through localized overheating.
Since a resistance paste prepared using general conductive carbon powder also has a negative temperature resistance factor of carbon, it is difficult to secure reliability due to reduction in resistance upon repeated use. Further, since a metallic material has a positive temperature resistance factor, it is difficult to secure reliability due to increase in resistance upon repeated use.
Korean Registered Utility Model No. 207322 discloses a car seat, which includes a cotton yarn or natural fibers as a warp, woven copper wires or natural fibers disposed in the same direction as the cotton yarn and separated a predetermined distance from each other, a heat generating yarn formed on the cotton yarns or natural fiber by carbon coating as a weft, a heating plate composed of upper and lower polyurethane coating layers, a temperature sensor attached to the heating plate to be turned on/off within a predetermined temperature range, and a connection terminal through which terminals of the copper wires are connected to a vehicle power supply.
Korean Registered Utility Model No. 300692 discloses a sheet type heating element, which is formed by screen printing and includes a bottom plate formed of a synthetic resin, a plurality of carbon paste lines formed on the bottom plate to provide a plurality of alternating ladder shapes, a plurality of silver paste lines connected to each other and each being deposited at one side of the carbon paste line or along an outer periphery of the carbon paste line to provide electrodes such that positive and negative electrodes alternate, a thin synthetic resin layer formed by coating and curing an insulating synthetic resin to a predetermined thickness and width on the carbon paste lines and silver paste lines, and a finishing plate formed of adhesive and bonding agents on the thin synthetic resin layer.
Korean Patent No. 644089 discloses a lumbar supporter which is provided as a back supporter of a vehicle seat and includes a heating wire embedded therein. The lumbar supporter includes a seat heat cushion and a seat heater back, each of which includes heat generating wires disposed on a plane of a heat resistant member and coupled at one side thereof to a connection jack to prevent disconnection of the wires due to user weight. In the seat heater cushion, a negative temperature coefficient (NTC) member is coupled to the other side of the heat generating wires to decrease resistance when the temperature of the heat generating wires increases. The NTC member is coupled at one side thereof to an electronic control unit (ECU) and a multi-stage variable regulator is coupled to one side of the ECU and the other side of the NTC member such that power is continuously turned on/off by resistance of the NTC and the regulator.
In the related art, although heating wires, carbon and the like are used as the heating element, carbon nanotube-based heating elements have yet to be introduced.